Page:The American Cyclopædia (1879) Volume V.djvu/192

 188 COMPASS COMPASS (SOLAE) 18". Since that time it has been slowly de- creasing. It is thus perceived that surveys made by the compass should always be referred to the true meridian, or their date be given, that such reference may at any time afterward be made; but the latter method is not al- together trustworthy. The subject of these movements for this continent is treated in the reports of the proceedings of the American as- sociation for the advancement of science, at their ninth and tenth meetings. The variation of the compass in ships from local causes is a matter of the most serious consequence, and baffles the skill of man to control its effects. It is not a constant determinable error, but varies with the position of the vessel ; it is generally greatest when she heads E. or W., and least when she lies on the magnetic me- ridian. It is different also as the ship lies on even keel or careens over on one side. The liability of error increases with the increased nse of iron in the construction of ships, and those built of it are found to act themselves as magnets, especially if they stood upon the stocks with the head toward the north. In this case, sailing for some time in a contrary direction tends to change their polarity and disturb the compasses. The plates are made magnetic by the blows they receive as the ship is constructed. The attention of scientific men has been directed for years past to the devising of some protection against this evil. In iron ships compasses are stationed in differ- ent parts, and comparative observations are constantly made of their indications. The most confidence is placed in those which are furthest from the hull, as at the mast head. In the British navy it has been the practice for many years to occasionally swing around each ship, and note the indications of the compass as she heads in different directions, and thus form a table of errors to be applied to cor- rect the compass when she is afterward sailing on these courses. The board of trade recom- mended the adoption of this practice for mer- chant ships. Professor Airy, astronomer roy- al, objected to it, and recommends instead the use of magnets placed near the compass, and so arranged as to neutralize the influence of all other local attractions. He also advises fre- quent examination of the compasses, and test- ing them as often as practicable by azimuth ob- servations of a star or other objects. In some ships a neutral point has been found in which the local attractions were all balanced, and so continued to be, rendering this a suitable spot for the compass. The discovery of a sure method of obtaining a neutral point of this character is an object of the highest conse- quence; but, however secured, there are so many causes to influence the condition of the needle, some slow and some sudden in their action, such as electrical currents induced by atmospheric agencies, the heavy shocks of the waves upon the ship, &c., that frequent observations and constant care are essential to make the compass a safe guide. Sir John Ross found that the needle was attracted full 5 by the rays of the moon concentrated upon it. The British admiralty have caused experi- ments to be made, and have invited compe- tition, in the construction of the compass 1 and in consequence, from being a common, it has become an artistic instrument in that country, great improvements having been made in it. COMPASS, Solar, an instrument invented by William A. Burt of Michigan, designed for de- termining at any place a true N. and S. line, from which other lines may be run in any de- sired direction. It is furnished with a latitude arc to be set at the degree of the latitude of the place, a declination arc to be adjusted to the degree of the sun's declination at the time, and an hour arc for setting off the hour of the Solar Compass. day. The latitude arc, &, has its centre of motion in two pivots, one of which is shown at d. It is moved up or down within a hollow arc by the tangent screw^j and reads to single minutes by the vernier e. It has a range of about 35, so as to be adjustable to the latitude of any place in the United States. The decli- nation arc is represented at &, which as well as the latitude arc is graduated to quarter degrees, and reads to single minutes by the vernier 0, which is upon the arm h with its centre of motion at ^, the centre of the arc. It is con- trolled by the tangent screw, seen near the arc. The hour arc, e, is supported by the two pivots of the latitude arc, and moves in a plane perpendicular to the declination arc. Through the centre of the hour arc passes a hollow socket p, in which the spindle of the declination arc turns. The hour arc is read by the lower edge of the graduated side of the declination arc. The combination of the declination arc (which is for the purpose of adjusting the sights, u, u, to the declination of the sun, so that its rays may be made to pass through them) and of the hour arc constitutes a very accurate form of equatorial sun dial, the arm h being made to turn upon an axis parallel with the axis of the earth, and to show the hour by indicating the number of degrees measuring the angle which the sun's rays make with the plane of the meridian. The in-